A comprehensive physiological and genetic analysis is proposed to study the biology of aging using rotifers as a model system. Physiological portions of the research are designed to quantify the influence of environmental stress on senescence rates. Using isogenic orthoclones of Asplanchna brightwelli and Brachionus plicatilis, age-specific rates of calcium accumulation will be measured and related to lifespan. Effects of maternal age at reproduction on progeny survivorship will be investigated in the same experiment. In addition, we seek to measure the effects on mean lifespan of population density, growth rate, and physiological stress in order to quantify the role of environmental factors in molding senescence patterns. Senescence can be related to both the amount of genetic variation and its packaging. We will perform a combined inbreeding and artificial selection packaging. We will perform a combined inbreeding and artificial selection analysis to examine the extent of genetic variation affecting survivorship, and to study the role of heterosis in determination of lifespan. Competition among clones with different fecundity and survivorship schedules will be used to investigate natural selection on length of lifespan. Finally, by comparing selection regimes promoting long lifespan versus early reproduction, we will experimentally test a theory of senescence based on accumulation of genes with pleiotropic effects (i.e., genes acting to both enhance early reproductive rate and decrease survivorship late in life).